1. Field of the Invention
The present invention relates to a process for preparing perfluorocarboxylic acid. In particular, it relates to a novel process for producing perfluorocarboxylic acid in two steps.
2. Description of the Prior Art
It is known that perfluorocarboxylic acids are strong acids having excellent heat and chemical resistance. Various uses of perfluorocarboxylic acids have been proposed because of the special surface activity of perfluoroalkyl groups. For example, ammonium salts and alkali metal salts of perfluorocarboxylic acids are indispensable emulsifiers for the polymerization of fluorinated olefins such as tetrafluoroethylene and the like. Perfluorocarboxylic acids are also used as vaporization suppressants for volatile combustible organic liquids. Perfluorocarboxylic acids are also useful as intermediates for the preparation of fluorocarbon surfactants having excellent surface activities. Certain metal salts and polymers derived from perfluorocarboxylic acids are also useful as water-repelling and oil-repelling agents.
There are several synthetic methods for preparing perfluorocarboxylic acids. Among them, is the well-known Simons electrolytic fluorination process which provides a simple means of obtaining the acid. ##STR1## However, the yield of the acid is very low because of the formation of cyclic ethers as well as of the cleavage of the carbon-carbon linkage.
Sulfur trioxide or chlorosulfonic acid can convert perfluoroalkyl iodide to the carboxylic acid. ##STR2## The handling of the highly corrosive reagents at high temperatures (160 - 200.degree. C.) is disadvantageous in large scale production.
The oxidation of perfluoroalkyl ethylene by potassium permanganate gives perfluorocarboxylic acid in fairly high yields. ##STR3## In this process manganese dioxide is also formed, in an amount equivalent to the acid formed. In this process it is difficult to specify the product and it is necessary to recover the manganese dioxide.
Accordingly, there is a need for a simple direct process for preparing perfluorocarboxylic acids.